In electronic timepieces currently found on the market it is usual to employ a stepping motor to convert electric impulses coming from a quartz time base into mechanical motion to display the time of day. The system is energized by an energy source, generally a battery of small dimensions, which must be periodically replaced. In order to spare the energy supplied by the battery, and thus to obtain the longest possible duration thereof, there have already been proposed regulating systems which slave the duration of the pulse driving the motor to the load which the motor must accommodate. In other words, the pulse is prolonged if the load increases and reduced if the load decreases. Such systems are described for instance in U.S. Pat. Nos. 4,323,834 and 4,346,463.
It is however necessary to distinguish among several types of loads acting on the rotor of the motor. Since a well defined angular position of the rotor is desired between steps it will be necessary to apply thereto a positioning rest couple which must be overcome each time that the rotor is to advance by a step. The rotor must likewise overcome various friction couples which are to be found in the bearings. Finally, the rotor will be required to supply a useful couple in order to drive the display mechanism. The energy to be supplied to the motor to overcome these different couples is generally well determined for a given type of watch which, when it operates normally, will consume a relatively constant amount of energy. Nevertheless, if the watch is provided with a calendar, it will be understood that once each 24 hours the useful couple to be supplied must be increased at the time of date change-over. It is then that slaving determined by the load may find use as mentioned hereinabove and of course in the situation where the watch is provided with a calendar or like system.
The slaving systems reacting to the imposed load on the rotor generally assume that the motor is energized at a constant voltage and do not take into account the variations between the voltage supplied by the battery at the beginning and then at the end of its life duration. At first glance this simplification may appear normal for a silver battery for which the voltages at the beginning and the end of life are respectively on the order of 1.6 and 1.4 volts. However, as will be seen, the difference indicated may lead already to an over-consumption by the system if measures are not taken in order to slave the length of the motor pulse to the voltage delivered by the energy source and likewise to the resistance presented by said source. It is evident on the other hand that if this difference increases still further as will be the case for instance in using lithium batteries, where the limits of operation may be fixed between 2.4 and 3.6 volts, the consumption in pure loss will be even greater.
There has come to our attention the European patent document EP No. 0 057 663 (corresponding to U.S. Pat. No. 4,439,717). This concerns a control arrangement for a stepping motor which sets out to avoid certain difficulties in the known arrangements arising from the fact that if the source voltage varies, the power supplied to the motor likewise varies, as set forth hereinabove. To relieve the difficulty the document suggests on the one hand energizing the motor with constant winding current and on the other hand analyzing the voltage signal present over the winding, then furnishing information on the voltage induced by the rotor movement.
The present invention is totally different inasmuch as no recourse is made to the induced voltage nor is there any necessity to maintain constant winding current. As will be subsequently seen the method now proposed is satisfied by measuring the energization voltage at the motor terminals and the winding current. The product of these quantities is integrated over a time period and the energy supply is cut off when the energy thus measured becomes equal to a predetermined quantity.